1. Field of the Invention
This invention relates to a process for catalytic cracking of a heavy-fraction oil. More particularly, it relates to a fluid catalytic cracking (FCC) process which comprises cracking a heavy-fraction oil to obtain light olefins such as ethylene, propylene, and butenes.
2. Description of the Prior Art
In the usual catalytic cracking technique, petroleum-derived hydrocarbons are catalytically cracked with a catalyst, thereby to obtain gasoline as the main product, a small amount of LPG, a cracked gas oil and the like, and coke deposited on the catalyst is then burnt away with air to recycle the regenerated catalyst for reuse.
In recent years, however, there has been a tendency that a fluid catalytic cracking apparatus is utilized not as an apparatus for producing gasoline, but as an apparatus for producing light-fraction olefins for use as petrochemical materials. Such utilization of an original fluid catalytic cracking apparatus as an olefin-producing apparatus is economically advantageous, particularly to an oil refinery which is highly integrated with a petrochemical industry.
On the other hand, much attention has been paid to environmental problems, and the regulation of the content of olefins and aromatics in gasoline for automobiles. Consequently, it can be anticipated that alkylate will be increasingly in demand as additive materials for high-octane gasoline, in place of FCC-produced gasoline and catalytically-reformed gasoline. Therefore, it will be necessary to increase the production of propylene and butenes which are raw materials for this additive.
Methods for producing the light-fraction olefins by the fluid catalytic cracking of heavy-fraction oils include methods which comprise contacting a feed oil with a catalyst for a short time (U.S. Pat. Nos. 4,419,221, 3,074,878, and 5,462,652, and European Patent No. EP 315,179A), a method which comprises carrying out a cracking reaction at a high temperature (U.S. Pat. No. 4,980,053), and methods which comprise using pentasil-type zeolites (U.S. Pat. No. 5,326,465 and Japanese Patent National Publication (Kohyo) No. Hei JP 7-506389).
Even these known methods still cannot produce sufficient light-fraction olefins selectively. For example, the high-temperature cracking reaction will result in a concurrent thermal cracking of heavy-fraction oils, thereby increasing the yield of dry gases from said oils. The short contact time of a feed oil with a catalyst will cause a decrease of conversion of light-fraction olefins to light-fraction paraffins due to its inhibition of a hydrogen transfer reaction, and it will be unable to increase conversion of heavy-fraction oils to light-fraction oils. Furthermore, the use of pentasil-type zeolites will only enhance the yield of light-fraction hydrocarbons by excessive cracking of the gasoline, once it is produced. Therefore, it is difficult to produce light-fraction olefins in a high yield from heavy fraction oils by using each of these known techniques alone.
An objective of this invention is to provide an improved process for the fluid catalytic cracking of a heavy-fraction oil, which can produce light-fraction olefins with a high yield, while producing a diminished amount of dry gases such as gaseous hydrogen, methane, and ethane generated by the thermal cracking (thermocracking) of the heavy-fraction oil.
In an attempt to mainly raise the yield of light-fraction olefins, in a process employing fluid catalytic cracking of a heavy-fraction oil at a high temperature and at a short contact time, the present inventors have found that this objective can be achieved by contacting the heavy-fraction oil with a catalyst mixture, that consists of a specific base cracking catalyst and an additive containing a shape-selective zeolite, at a high temperature. This invention has been achieved on the basis of this finding.
More particularly, the process for fluid catalytic cracking of a heavy-fraction oil according to this invention comprises the step of contacting the heavy-fraction oil with a catalyst mixture, consisting of 60 to 95 wt % of a base cracking catalyst containing an ultra stable Y-type zeolite and less than 0.5 wt % of rare-earth metal oxide, and 5 to 40 wt % of an additive containing a shape-selective zeolite; the oil and the catalyst are contacted in a fluid catalytic cracking apparatus having a regeneration zone, a down flow-type reaction zone, a separation zone, and a stripping zone, and are contacted under conditions so that the reaction zone outlet temperature is in the range of 580 to 630xc2x0 C.; the catalyst/oil ratio is in the range of 15 to 40 wt/wt, and the contact time of hydrocarbons in the reaction zone is in the range of 0.1 to 1.0 seconds.
This invention will be described below in more detail.
Feed Oil
In the fluid catalytic cracking of this invention, a heavy-fraction oil is used as a feed oil. The heavy-fraction oil used preferably has a boiling point in the range of 250xc2x0 C. or higher, at atmospheric pressure. The heavy-fraction oil used herein may include straight-run gas oil, vacuum gas oil, atmospheric residue, vacuum residue, coker gas oil, or heavy-fraction oils obtained by hydrofining said residues and gas oils. These aforementioned heavy-fraction oils may be used singly or jointly, or as a mixture thereof, with a minor portion of a light fraction oil.
Apparatus and Process
The fluid catalytic cracking apparatus which can be used in this invention has a regeneration zone (a regenerator), a down flow-type reaction zone (a downer reactor), a separation zone (a separator), and a stripping zone (a stripper). The term xe2x80x9cfluid catalytic crackingxe2x80x9d referred to herein indicates that the above described heavy-fraction oil, as the feed oil, is continuously brought into contact with a catalyst which is kept in a fluidized state under specific operating conditions, to crack the heavy-fraction oil, thereby producing light-fraction hydrocarbons, mainly comprising gasoline and light-fraction olefins.
In the reaction zone, the fluid catalytic cracking may be effected within a fluid bed, in which the catalyst particles are fluidized with the heavy-fraction oil, or, may be effected by employing so-called riser cracking, in which both the catalyst particles and the heavy-fraction oil ascend through a pipe, or, so-called down flow cracking in which both the catalyst particles and the heavy-fraction oil descend through a pipe. In case of riser cracking in a riser reactor, however, back-mixing of hydrocarbons takes place in the reactor, which causes localized long residence times of hydrocarbons in the reactor, resulting in enhancement of thermal cracking. Particularly, in fluid catalytic cracking processes, like this invention, operated with higher reaction temperatures than ordinary fluid catalytic cracking processes, thermal cracking caused by back-mixing is significant. With such an increase in the contribution of thermal cracking, the undesirable dry gases increase and the desirable gasoline and light-fraction olefins decrease. In this invention, therefore, the down flow-type reaction zone is employed to avoid thermal cracking.
The fluid catalytic cracking process of this invention will be described in detail. First, in the reaction zone, the heavy-fraction oil is continuously brought into contact with the catalyst mixture which is maintained in a fluidizing state, under the following specific operating conditions, to crack the heavy-fraction oil, thereby producing light-fraction hydrocarbons, mainly comprising light-fraction olefins. Then, a mixture of the catalysts and a hydrocarbon gas, comprising products (cracked products) obtained by the catalytic cracking, mixed with un-reacted materials, is forwarded into the separation zone, in which most of the catalyst is separated from the hydrocarbon gas.
Next, the separated catalysts are forwarded to the stripping zone, in which most of the heavy hydrocarbons comprising some of the products, and the un-reacted materials, which are adsorbed on the catalysts, are removed from the catalyst particles. The catalysts on which carbonaceous materials, and a portion of heavy hydrocarbons, are deposited, are forwarded from the stripping zone to the regenerating zone. In the regenerating zone, the catalysts, on which the carbonaceous materials and the like are deposited, are subjected to oxidation treatment, to decrease the amount of the deposits, thereby obtaining regenerated catalysts. These regenerated catalysts are continuously recycled to the reaction zone. In one particular case, the cracked products are quenched just upstream of, or just downstream of, the separator, in order to avoid unnecessary further cracking or excessive cracking.